56 ORIGIN OF CHORDATES m. 2 



Perhaps the most interesting behaviour observed was the activity 

 shown by an isolated proboscis, collar, trunk, or portion of trunk. 

 These organs may move around vigorously in an exploratory manner; 

 evidently the main nerve-cords are not necessary for the initiation of 

 action, as is the central nervous system of higher chordates. 



There are nerve-fibres in the walls of the pharynx and oesophagus, 

 where peristaltic movements have been observed. Their relationship 



an. 



Fig. 29. Young tornaria larva, seen from the side. 



an. anus; ap. apical organ; cb. longitudinal ciliated band; m. mouth; 



pb. posterior ciliated band; pp. proboscis pore. (After Stiasny.) 



to the rest of the nervous system is unknown. They may represent 

 the beginnings of an autonomic nervous system. 



The sexes are separate in enteropneusts and the gonads resemble 

 those of amphioxus in being a series of sacs developing from cells just 

 outside the coelom. These proliferate and bulge into the coelom, 

 covered by the somatopleure. They acquire a cavity and each opens 

 by a narrow duct to the exterior, fertilization being external. The 

 development is remarkably like that of echinoderms. Cleavage is holo- 

 blastic and resembles that of amphioxus and ascidians, gastrulation is 

 by invagination, and the coelom is formed as an enterocoele, later 

 becoming subdivided into proboscis, collar, and trunk coeloms. Hatch- 

 ing occurs to produce a pelagic tornaria larva, with a ciliated band that 

 has exactly the relations found in the dipleurula larva of echinoderms. 

 The band passes in front of the mouth, down the sides of the body, 

 and in front of the anus (Fig. 29). It then divides into more dorsal 



